The present invention relates to methods and units for vaporizing liquids, particularly for distilling air.
All gaseous mixtures that are to be separated, and in particular atmospheric air, contain numerous and varied impurities of varying volatility and in highly varying contents. Not being readily soluble, these impurities, alone or in combination, present risks of malfunctioning, particularly of explosion.
In the case of air-distillation apparatus, atmospheric air to be separated is purified prior to liquefaction in an adsorption-type purification unit which holds back almost all of the water, carbon dioxide and unsaturated hydrocarbon impurities present in the air.
This prepurification is not, however, sufficient, and it is therefore envisaged that it be supplemented by a system for reducing the impurities concentration by continuous cleaning and/or adsorption in the liquid phase, as described, for example, in document DE-A-1, 936,049.
What is more, when a perfectly identified and detectable difficult-to-remove impurity may pose a problem, then the measuring of the presence of this impurity in a critical region of the apparatus is advantageously envisaged, as described, for example, in the case of the impurity N2O in document U.S. Pat. No. 5,629,208 (Darredeau et al.).
It is nonetheless still the case that many impurities, for example temporary, local or unpredictable impurities, are not taken into account, existing purification devices allowing some of these to be held back but being relatively transparent in the case of others which, depending on their limits of solubility and of concentration, may be deposited on the heat-exchange surfaces of the apparatus, particularly the vaporizers.
To avoid this risk, use is made of bath-type vaporizers in which a thermosiphon allows a high flow rate of liquid to be circulated through the exchanger so as to thoroughly wash these surfaces. Use is also made of vaporizers known as falling film or liquid-film wetting vaporizers, in which the liquid to be vaporized is finely distributed at the upper part of the surface and trickles over this surface in thin films allowing a smaller temperature difference. To avoid the deposition of solid particles on the surface, care is taken to ensure that this surface is kept very wet, right down to the bottom of the vaporizer, by supplying it with a flow rate of liquid that is markedly higher than the flow rate vaporized, usually by resorting to the use of a recirculating pump. The latter does, however, have the drawback of concentrating the residual impurities in the liquid bath, that the filtration device in the aforementioned recirculation circuit may not be adequate to hold back.
The object of the present invention is to propose methods and devices that will make it possible, not to detect the presence of impurities in the liquid that is to be evaporated, but to detect any formation of depositions of impurities on the critical heat-exchange surfaces and therefore take rapid action to change the operating conditions of the plant and/or provide greater purification, at least temporarily, and/or shut down the plant.
In order to achieve this, according to one aspect of the invention, this invention proposes a method of detecting the presence of residual impurities in at least one stream of liquid intended to be at least partially vaporized in at lest one vaporizing unit, in which part of the stream entering the vaporizing unit is diverted so that at least part of it is directed onto a receiving surface that is held at a temperature slightly above the vaporization temperature of the liquid being monitored, and in which any formation of solid deposits on the surface is detected.
Typically, according to the invention, the amount of solid material deposited and the amount of diverted liquid vaporized on the surface are also measured, so as to establish, from correlations, the presence, nature and content of impurities during the period preceding these measurements.
Thus, whatever the quality of the purification operations performed upstream or locally, the absence of the formation of solid deposits on the receiving surface, which reconstructs on a small scale and in a directly accessible manner, the situation of the heat-exchange surface within the apparatus, demonstrates that under the prevailing operating conditions, the risks of the deposition or build up of undesired solid particles on the heat-exchange surface are zero. By contrast, whatever the impurity involved, whether or not it has been previously identified, the appearance of a deposition on the receiving surface indicates a risk condition that allows corrective measures to be taken swiftly.
Another subject of the present invention is a vaporizing unit that is appropriate for the implementation of the method, comprising, within an enclosure, at least one vaporizer, means of supplying the vaporizer with a stream of liquid to be vaporized, a circuit for diverting part of the stream to be vaporized, this circuit being connected to the supply means and comprising a means of spraying part of the diverted stream onto a receiving surface and means of detecting the formation of solid deposits on this surface.
Such a vaporizing unit can be readily implemented in numerous applications, possibly by making quick modifications to existing vaporizing units. It finds a particularly advantageous application in vaporizer-condensers at the base of the upper columns of double-column distillation devices, with simple gravity feed and/or fed at least partly by a recirculating pump.
Real-time checks for the absence of formation of deposits (or, in the other case, the appearance of such deposits) makes it possible to operate under perfectly reliable conditions, without having to resort to preventive measures that are not necessarily needed such as, for example, overwetting the vaporizer or running it at a high pressure. Thus, according to one aspect of the invention, it is possible to vaporize oxygen at a pressure not exceeding 3.5xc3x97105 Pa and to produce a double air-distillation column comprising, at the base of the upper column, a liquid-film wetting vaporizer that is entirely devoid of a recirculating pump.